The present invention generally relates to devices used to physically support the heart and, alternatively, also actively assist the pumping action of the heart.
The treatment of heart failure over the long term is a difficult problem. At the same time, weak cardiac muscle function is becoming an increasing problem. Patients are surviving longer and more patients are surviving myocardial infarcts leading to a large pool of patients who are inadequately served by current medical practice. Drug treatment to increase the strength of mycardial contraction has been unsuccessful over the long term. Recently, biventricular pacing (rather than the usual univentricular pacing) has been tried and this offers some promise in selected patients but is unlikely to solve the problem.
Devices will therefore remain the mainstay of treatment for terminal heart failure. Conventional methods have been unable to inject adequate energy into the cardiovascular system. Past attempts with the Jarvic heart or other replacement systems have met with problems such as failure due to thromboembolism. The patient is typically connected to a bulky internal or external controller and power supply for the heart replacement system. The inside of the artificial heart exposes a large artificial surface area to the flow of blood and clots develop as a result. These clots eventually break off and lodge in the brain leading to strokes or resulting in ischemic injury to other body organs. It has also been postulated that long-term exposure of blood to large artificial surfaces sets up a chronic inflammatory reaction which may predispose the patient to infection.
Currently, there are two major areas of development. A simplified system involves cannulation of the left ventricle or atrium with a tube-like structure and pumping of blood from this source into the aorta. A blood propeller system is located within the tubing of this system. A drive system powers the pump. The drive system can be located outside the patient, or can be implanted within the patient. If implanted, energy may be transmitted by induction coils from outside the body to the device. This device requires considerable residual cardiac function to operate. The heart must beat adequately to perform some function and usually only the left ventricle is supported by the device. Thus, right ventricular function must be adequate for survival.
The second and more complex pump is a totally implantable heart. The patient""s heart is entirely removed or both ventricles are cannulated and artificial left and right ventricles are attached by a surgeon. The patient has a large surface exposed to the flow of blood as the blood comes in contact with the artificial ventricles, the connection tubes and the valves. Blood clotting, hemolysis and degradation of blood become problems in this situation.
For an entire generation, attempts have been made to create a heart assist device which leaves the native heart in place and squeezes the native heart. The blood is thus exposed only to the patient""s natural tissue. Clotting on natural tissue is extremely rare. Pneumatically and electrically driven devices have been evaluated, but these devices have not reached clinical application. These devices have wrapped around the entire heart and squeezed both the left and right ventricles. Unfortunately, this does not mimic the way the heart contracts.
U.S. Pat. No. 4,925,443 illustrates a heart assist device including a tension band which is surgically placed within an interventricular muscle wall in order to compensate for weakness of the interventricular muscle wall or septum. An operating mechanism then opens and closes a pair of pressure plates to compress the left ventricle. The drawback to this device, however, is that the interventricular wall or septum experiences significant trauma due to the surgical implantation of the band within the wall or septum itself. Especially in cases in which the interventricular wall is already weakened, such trauma could severely damage the heart.
Another proposed device is disclosed in U.S. Pat. No. 5,119,804. With this device, the heart is placed within a cup having a vacuum source connected to hold the cup in position around the heart and having a pulsed pressure system to alternately apply relatively high positive and negative pressures to provide systolic and diastolic effects on the heart. This system, however, squeezes the entire heart muscle at one time and will tend to cause weaker portions of the heart to bulge outward while stronger portions of the heart muscle retain a normal shape. Therefore, the contraction applied to the heart muscle is not a natural one, but one that is dictated by the particular heart problems of the patient.
Another ventricular assist device is disclosed in U.S. Pat. No. 4,685,446. This device utilizes an inflatable balloon secured to the end of a catheter and inserted into the left ventricle. The balloon is inflated during left ventricular systole and then deflated in a repeating manner. Unfortunately, this device will also tend to cause weakened portions of the heart muscle to bulge around the left ventricle rather than causing the intended function of expelling blood from the ventricle. Thus, the ejection fraction of blood can be deficient with this device as well.
Despite the intuitively attractive nature of heart assist devices, no device has ever been clinically proven. Attention to some physiologic details will make the difference. The left ventricle is a thick-walled structure which propels blood into the systemic circulation at high pressure. The left ventricle is shaped as a truncated cone. During systole (contraction) this cone shortens along its length and narrows around its circumference. By this narrowing and shortening action, the internal volume of the left ventricular cavity decreases and blood is expelled. In a healthy heart, 60% to 70% of the blood volume (that is, the ejection fraction) is expelled on each beat. As the heart fails, the cavity enlarges, the heart wall thins and progressively smaller fractions of blood are expelled on each beat. In other words, the heart shortens and narrows much less during each beat.
The right ventricle has been described as a bellows pump. It wraps around and attaches to the circumference of the outside of the left ventricular wall. The outside wall of the right ventricle is considerably thinner than the wall of the left ventricle and also contracts against a lower pressure. The energy consumption of the right ventricle is therefore much lower than that of the left ventricle. The right ventricle expels blood when the muscle shortens and reduces the diameter of the crescent shaped cavity which is located between the outside wall the interventricular wall or septum shared with the left ventricle.
It is not surprising that merely squeezing the left and right ventricles with a device wrapped around both ventricles has not been successful. With previous devices, the left ventricle does not shorten from base to apex. There is also limited short axis shortening because the device does not squeeze the left ventricle in isolation, but with the right ventricle. To be effective the left ventricle requires more controlled compression. Generally, blood must be expelled from the ventricle in a more controlled and complete manner.
The present invention is generally directed toward heart support and assist devices including fully passive restraints, combinations of passive and active devices and fully active devices for assisting with heart contractions. Passive restraints generally include an external support member, which may be a strap, web or mesh, sheathing or other member configured to extend around the outside of the heart coupled with an internal support member extending within at least one of the ventricles and against one side of the interventricular septum. This type of passive restraining system can assist the heart muscle by supporting those portions of the muscle necessary to produce efficient contractions either naturally or with another active assist device. This support is provided in a manner that minimizes trauma to the heart muscle. Additional internal tensile members, such as cables, may be connected to the external tensile member or members longitudinally and/or transversely through one or both ventricles. These cables will assist with long axis and short axis shortening of the heart muscle during each contraction.
Combinations of passive and active devices may include, for example, external support members, in the form of straps, sheaths, wraps, mesh elements or webs, etc., combined with a blood pump connected for fluid communication directly with the left ventricle, right ventricle or both. Alternatively, a fluid inflatable bladder may be placed between the external tensile member and the outside surface of the heart to provide compression to one or both of the ventricles to assist in pumping blood through the heart. Finally, an active contraction device may integrate an external tensile member system with a powered actuator device to provide cyclical compression of the heart muscle through a pulling action on the tensile member or members.
In another aspect, the invention is directed to a heart assist device generally including a plurality of flexible tensile members adapted to be wrapped circumferentially about the heart of a patient. At least one tensile member is configured to extend around the left ventricle and a second tensile member is configured to extend around the right ventricle. A support member is configured to be received within the right ventricle against the interventricular septum and coupled to at least one of the first and second tensile members. This support member may be a portion of at least one of the tensile members or may be a separate member connected to at least one of the tensile members. At least one powered actuator may be operatively connected with the first and second tensile members and operates to pull the tensile members respectively against the left and right ventricles to expel blood therefrom.
More preferably, the heart assist device includes a plurality of tensile members configured to extend around the left ventricle and a plurality of tensile members configured to extend around the right ventricle. Each tensile member is secured at least indirectly to the support member. The support member is preferably a plate covered with a biocompatible material for inhibiting blood clotting. The actuator pulls the tensile members extending around the left ventricle against the outside surfaces of the heart and pulls the support member or plate against the interventricular septum in an opposing direction. The tensile members extending around the right ventricle are pulled against the left ventricle in an independent fashion.
One preferred embodiment of the invention may include a plurality of pulley members coupled with the tensile members and operating to allow a single powered actuator, such as an electric or pneumatic actuator, to pull multiple tensile members. Alternatively, multiple powered actuators may be used to independently pull the various tensile members. The tensile members, pulleys and other actuating structure may be contained in a suitable jacket or sheath positioned around the heart.
In accordance with another aspect of the invention, at least one internal tensile member is provided and configured to be connected lengthwise within the left ventricle between the mitral valve of the heart and the apex of the left ventricle. The internal tensile member inhibits lengthening of the ventricle when the powered actuator or actuators pull the tensile members to compress the left and right ventricles. As further options, transverse, internal tensile members may be connected within the left ventricle between the outside wall thereof and the interventricular septum to control widthwise expansion. Also, one or more internal tensile members may be utilized in the right ventricle for similar purposes.
As additional aspects of the invention, the tensile members may be contained in sleeves to prevent cutting of the heart by the tensile members during use. Also, a plurality of coronary obstruction preventing members may be used between the tensile members and the coronary arteries on the outside of the heart for preventing the coronary arteries from being compressed and obstructed by the tensile members.
The present invention also generally contemplates methods for assisting the pumping action of the heart. In a preferred embodiment, the method includes inserting an anchor member within the right ventricle and against the interventricular septum; encircling the outside of the right and left ventricles with respective tensile members; coupling the tensile members with the anchor member; and compressing the right and left ventricles by pulling the tensile members against the outside of the heart. Other methods will be apparent to those of ordinary skill based on a full review of this disclosure.
In various aspects of the invention, a basic device for assisting a heart may comprise a plurality of flexible, external tensile members adapted to be wrapped circumferentially around the heart of a patient. Preferably, this includes at least a first external tensile member configured to extend around the left ventricle and a second external tensile member configured to extend around the right ventricle. In accordance with the invention, an internal support member is configured to be received within at least one of the left and right ventricles and against the interventricular septum. This support member is coupled either directly or indirectly to at least one of the first and second external tensile members. The internal support member may comprise a portion of one or more of the external tensile members or may be a separate member, such as a plate, coupled with the external tensile members. The external tensile members are preferably flat straps or other similar structures that will not harm the outside of the patient""s heart, and may be formed from any biocompatible material. In various embodiments, the support members may be implanted either partially or completely through one or more catheters.
In another embodiment of the invention, at least one of the first and second external tensile members may be configured generally in a spiral shape to facilitate the application of compression to the heart. In this embodiment, for example, one or more coils of the spiral may extend into one of the ventricles of the heart and bear against one side of the interventricular septum to form a support member as described above. An actuator is used to draw the spiral-shaped external tensile member into a tighter, coiled shape to actively compress or passively support one or both ventricles of the heart.
In another embodiment of the invention, the first and second external tensile members are configured as first and second halves of a cup. The cup is configured to envelop the patient""s heart and comprises first and second shells with at least a first bladder configured for disposition between one of the shells and an outside surface of the heart. As with the other embodiments, one or more support members extend between opposite sides of the cup and within one or both ventricles of the heart to bear against the interventricular septum. A pump is provided for selectively inflating and deflating the bladder to apply compression to at least one of the left and right ventricles. In the preferred embodiment, a bladder is connected within each of the shells associated with the cup for compressing both of the ventricles. The support member or members are connected at a position generally between the first and second halves of the cup such as by being retained in place by the same connectors used to affix each half of the cup together. The cup may be formed in one, two or more pieces and, again, is formed from any suitable biocompatible material or materials as with all of the implantable components of each embodiment.
Various objectives, features and advantages of the invention will become more readily apparent to those of ordinary skill in the art upon review of the following detailed description taken in conjunction with the the accompanying drawings.